This invention relates to optical measuring systems, and more particularly to zoom lens systems of the type which are employed in the field of optical dimensional metrology. More specifically, this invention relates to a novel method and means for calibrating a zoom lens system of the type described so that a selected or given magnification or field of view (FOV) of the lens system can be accurately re-established repeatedly without requiring re-calibration of the system from an external reference source.
One of the shortcomings of known optical systems of the type that employ a zoom lens for metrological applications is the difficulty in repeatedly and accurately resetting the lens in order to re-establish a particular magnification or field of view which may have been used previously during the course of conducting an optical measurement of a workpiece. This shortcoming results from the fact that the instrument's zoom lens typically is adjusted by a mechanical device, and consequently calibration of the magnification of the lens can be altered because of, for example, backlash in the mechanics of the zoom mechanism, wear of mechanical parts during useful life of the instrument, positioning error resulting from the electro-mechanical system which drives the zoom mechanism, and even thermal effects on the glass and metal parts of the system. It is, however, particularly desirable to be able accurately to re-establish one or more such magnifications in order to allow accurate measurement of a workpiece.
For example, the assignee of this application has heretofore marketed under the trademark SMART SCOPE an optical measuring system of the type in which a zoom lens is utilized to project the image of a workpiece to a video camera, which in turn projects the image via a microprocessor onto a monitor. To record measurements of the workpiece, the operator then shifts the workpiece holder laterally, and in a manner to cause the image of the workpiece likewise to be shifted laterally relative to a fixed scale on the monitor, thus to measure the image via such scale. For subsequent workpieces the operator again manipulates the zoom mechanism via a mouse or joystick which forms part of the SMART SCOPE equipment until an image of the new workpiece appears to be in proper focus on the monitor, after which the new workpiece can be manipulated, as described above, to effect the desired measurements.
One of the advantages of being able accurately to reestablish a previously employed magnification, rather than simply relying upon observation of the image of a workpiece on the monitor, is that it enables measurements of a workpiece to be made directly by the equipment without having to manipulate the workpiece to effect movement of its image relative to a scale on the monitor.
Accordingly, it is an object of this invention to obviate the above-noted shortcoming by providing apparatus for projecting and recording, for a selected magnification, an optical image of known dimension, so that when it is desired once again to adjust an associated zoom lens to the former (now stored) magnification, the optical image of known dimension can be used for accurately reestablishing the magnification.
Another object of this invention is to provide a novel method for obviating the above-noted shortcoming of a zoom lens system, by projecting a reference or reticle image into the zoom lens setting for a given or selected magnification, and electronically recording and storing the optical image for use at a later date to re-establish the former magnification setting.
Other objects of the invention will be apparent hereinafter from the specification and from the recital of the appended claims, particularly when read in conjunction with the accompanying drawings.